Scaling artifacts and resolution

A crash course in sampling theory

Whenever an image is rescaled or resampled, it will suffer from one or
more of the following artifacts:

Aliasing / moire

Loss of frequency response / blurriness

Ringing / halos

Original

Resized 2X smaller using Photoshop's Nearest Neighbour algorithm

The original test pattern consists of a zone plate (the image with the concentric
circles) and sample text. When resized using the Nearest Neighbour algorithm,
aliasing can be seen as the spurious bands that appear in the zone plate test
pattern.

One way to remove aliasing is to use a blurry filter upon resampling (i.e.
a filter with poor frequency response). Of course, the problem with this approach
is that it makes the image blurry! This can be seen below:

Gaussian blur applied before resizing.

There are other resampling approaches that can create sharp images with low
aliasing. However, these filters will introduce ringing artifacts,
which can be seen as the halos around the edges of the text as shown below.
On highly saturated objects, these 'halos' will be of the opposite color of
the text.

Original.

Resized 2X smaller using Photoshop's Bicubic Sharper
algorithm.

Image on the left zoomed 200% larger.

No matter what resampling algorithm is used, there is always a tradeoff. This
dilemma can be visualized as picking a point inside the following triangle.
Improving one aspect (i.e. moving away from one corner) will make the other
artifacts worse.

In a broadcast monitor, resampling should be avoided where possible! Some manufacturers
sell LCDs panels that do not have at least 1920 x 1080 pixels. If you need to
monitor a 1920 x 1080 HD image, the image must be resampled to fit the whole
image onto the display. Not only will it be incapable of displaying full HD
resolution, there will be scaling artifacts on top of that resolution
loss. For critical monitoring, it is highly undesirable for the monitor to introduce
artifacts which do not exist in the image. Considering that there are many affordable
1920 x 1080 monitoring solutions, there is little reason to buy a reference
display that cannot do this. The one exception would be if you only needed to
monitor 720p footage and the display in question has sufficient pixels. However,
most people need to work with multiple formats including 1920x1080 formats (e.g.
1080i59.97, 1080i50, 1080psf24, and the next-generation 1080p50/59.97 HD formats).

When monitoring scaled signals on a LCD monitor (e.g. upconverted 720p or SD),
you should be aware that the monitor may introduce subtle scaling artifacts
not in the footage. This can be avoided for 720p footage by setting the monitor
to do a 1:1 pixel mapping to avoid scaling. For SD footage, all footage must
be scaled since SD uses non-square pixels (the pixels must be scaled into square
pixels to match the square pixels of the display). A 1:1 mapping mixing square
and non-square pixels would cause the aspect ratio to be wrong.

The CRT

CRT monitors are not limited to a fixed pixel structure and therefore do not
have to resample the image like LCDs. They can display the various HD and SD
formats (both anamorphic and 4:3) without scaling artifacts. They can also display
the non-square pixels of SD formats correctly without resampling, unlike LCDs
and other display technologies with a fixed pixel structure.

However, CRTs tend to have a luminance drop for very fine vertical detail due
to the left-to-right scanning of the electron beam. This can be seen in the
dark vertical band where the red arrows are. (The photo has some aliasing artifacts
due to the way the picture was taken and resized. Please ignore them.)